Difference between train and a normal GA? O.o

Simple answer: it is more fun :)

several reasons, but top few i can think of:

just for fun / uniqueness
to increase the private portion of sgtools pie / maintain a personal preference ratio
to farm for whitelists / create attention towards yourself
celebration containing more then one GA xD

edit: forgot two of the most important ones:
to keep people who are to lazy to click next a few dozen times, out of the good stuff at the end of a train.
for sgtools protection

• only one link to enter (easy to save and enter giveaways later)
• chance to find some "hidden gems"
• interesting giveaway description

For the convenience of the user. And yeah, it's fun.

Its allows you to control who you want to enter your giveaways as well as what's mentioned above